1. Field of the Invention
This invention relates to drug delivery systems, the prevention of post-surgical adhesions, ophthalmic corneal protective devices, and a surgical device used in the correction, for instance, of corneal ulcers, irregularities, scarring, astigmatism, myopia, and hyperopia.
2. Description of the Prior Art
Over the years, methods have been developed to achieve the efficient delivery of a therapeutic drug to a mammalian body part requiring pharmaceutical treatment. Use of an aqueous liquid which can be applied at room temperature as a liquid but which forms a semisolid gel when wanned to body temperature has been utilized as a vehicle for drug delivery since such a system combines ease of application with greater retention at the site requiring treatment than would be the case if the aqueous composition were not converted to a gel as it is warmed to mammalian body temperature. In U.S. Pat. No. 4,188,373, PLURONIC.RTM. polyols are used in aqueous compositions to provide thermally gelling aqueous systems. Adjusting the concentration of the polymer provides the desired sol-gel transition temperature, that is, the lower the concentration of polymer, the higher the sol-gel transition temperature, after crossing a critical concentration minimum, below which a gel will not form.
In U.S. Pat. Nos. 4,474,751; '752; '753; and 4,478,822 drug delivery systems are described which utilize thermosetting gels; the unique feature of these systems is that both the gel transition temperature and/or the rigidity of the gel can be modified by adjustment of the pH and/or the ionic strength, as well as by the concentration of the polymer.
Other patents disclosing pharmaceutical compositions which rely upon an aqueous gel composition as a vehicle for the application of the drug are U.S. Pat. Nos. 4,883,660; 4,767,619; 4,511,563; and 4,861,760. Thermosetting gel systems are also disclosed for application to injured mammalian tissues of the thoracic or peritoneal cavities in U.S. Pat. No. 4,911,926.
Ionic polysaccharides have been used in the application of drugs by controlled release. Such ionic polysaccharides as chitosan or sodium alginate are disclosed as useful in providing spherical agglomerates of water-insoluble drugs in the Journal of Pharmaceutical Sciences volume 78, number 11, November 1989, Bodmeier et al. Alginates have also been used as a depot substance in active immunization, as disclosed in the Journal of Pathology and Bacteriology volume 77, (1959), C. R. Amies. Calcium alginate gel formulations have also found use as a matrix material for the controlled release of herbicides, as disclosed in the Journal of Controlled Release, 3 (1986) pages 229-233, Pfister et al.
In U.S. Pat. No. 3,640,741, a molded plastic mass composed of the reaction product of a hydrophilic colloid and a cross-linking agent such as a liquid polyol, also containing an organic liquid medium such as glycerin, is disclosed as useful in the controlled release of medication or other additives. The hydrophilic colloid can be carboxymetnyl cellulose gum or a natural alginate gum which is cross-linked with a polyol. The cross-linking reaction is accelerated in the presence of aluminum and calcium salts.
In U.S. Pat. No. 4,895,724, compositions are disclosed for the controlled release of pharmacological macromolecular compounds contained in a matrix of chitosan. Chitosan can be cross-linked utilizing aldehydes, epichlorohydrin, benzoquinone, etc.
In U.S. Pat. No. 4,795,642, there are disclosed gelatin-encapsulated, controlled-release compositions for release of pharmaceutical compositions, wherein the gelatin encloses a solid matrix formed by the cation-assisted gellation of a liquid filling composition incorporating a vegetable gum together with a pharmaceutically-active compound. The vegetable gums are disclosed as polysaccharide gums such as alginates which can be gelled utilizing a cationic gelling agent such as an alkaline earth metal cation.
While the prior art is silent with respect to aqueous drug delivery vehicles and isotonicity thereof, osmotic drug delivery systems are disclosed in U.S. Pat. No. 4,439,196 which utilize a multi-chamber compartment for holding osmotic agents, adjuvants, enzymes, drugs, pro-drugs, pesticides, and the like. These materials are enclosed by semipermeable membranes so as to allow the fluids within the chambers to diffuse into the environment into which the osmotic drug delivery system is in contact. The drug delivery device can be sized for oral ingestion, implantation, rectal, vaginal, or occular insertion for delivery of a drug or other beneficial substance. Since this drug delivery device relies on the permeability of the semipermeable membranes to control the rate of delivery of the drug, the drugs or other pharmaceutical preparations, by definition, are not isotonic with mammalian blood.
Corneal protective devices are needed in cases in which corneal injury occurs and the immobilization of the eye using an eye patch is not resorted to. Molded collagen shields have been developed for this use. These are often not satisfactory because they lack sufficient flexibility to adequately conform to the individual corneal curvature. Wetting a collagen shield will increase conformance of the shield to the cornea but fragmentation can occur upon exceeding the flexibility of the collagen shield. The clinical uses of collagen shields are disclosed by Poland et al. in Journal of Cataract Refractive Surgery, volume 14, September 1988, pages 489-491. The author describes the use of collagen shields immersed in tobramycin solution in order to rehydrate the collagen prior to use. These are described as useful following cataract extraction or in patients having nonsurgical epithelial healing problems. More rapid healing of epithelial defects after surgery is resulted from the use of the collagen shield. Collagen shields have also been utilized as agents for the delivery of drugs to the cornea as disclosed in Reidy et al Cornea, in press, 1989 the Raven Press Ltd., New York and Shofner et al, Opthalmology Clinics of North America, vol. 2, No. 1, March 1989, pages 15-23.
Refractive surgery has been promoted in the United States and Russia over the past few years but its acceptance has been limited because of the poor predictability of the final optical results which include a resulting glare from incisions that encroach upon the optical zone. Techniques that rely upon the surgical production of corneal incisions have yielded inconsistent results because these surgical incisions in the cornea have been found to vary considerably in depth and length.
Laser keratectomy has been shown to be capable of yielding a more accurately controlled depth of corneal excision since each individual laser pulse excises a specific amount (0.2 to 10.0 um) of corneal tissue. Accordingly, the depth of excised tissue is in theory uniform and predictable, provided that the energy distribution is homegeneous across the laser beam. Since the primary locus of astigmatism is in the cornea, surgical intervention for astigmatism is more important than for the correction of other refractive errors, especially since spectacle or contact lens correction is of limited value in compensating for large astigmatic errors.
The excimer laser was introduced to ophthalmology in 1983 (Trokel, S., et al., "Excimer surgery of the cornea," Am. J. Ophthalmol. 96: 710-715, 1983). The depth of incision with short intense pulses permitted great precision to be achieved in tests on freshly enucleated cow eyes. The photochemical laser-tissue interaction is not thermal, permitting direct breaks of organic molecular bonds without involving optical breakdown in adjacent tissue. Early experimental results in rabbits revealed problems of 1) corneal stromal swelling, probably in response to disturbed water relationships due to compromise of the epithelial barrier and severing of the lamellae and (2) rearrangement of endothelial cells resulting from loss of contact inhibition (Marshall, J., et al., "An ultrastructural study of corneal incisions induced by an excimer laser at 193 nm", Ophthalmology 92: 749-758, 1985). Experiments with freshly enucleated human eyes indicates that flattening obtained by excimer laser ablation correlated with results of clinical scalpel radial keratotomy, but evaluation of the effects on wound healing and possible damage to adjacent structures was not addressed (Cotlier, A. M., et al., "Excimer laser radial keratotomy," Ophthalmology 92: 206-208, 1985). It was, however, suggested that this laser may become very useful in applications including penetrating and lamellar keratoplasty, keratomileusis, and epikeratophakia. Control of the area and depth of pulses using photolithographed masks resulted in ability to produce narrow cuts (20 um) and at depths depending on pulse number (Puliafito, C. A., et al., "Excimer laser ablation of the cornea and lens", Ophthalmology 92: 741-748, 1985). These controlled ablations had only very narrow bands of destruction at the adjacent edges. These studies led to the quantitation of laser ablation (Kruegar, R. R. and S. L. Trokel, "Quantitation of corneal ablation by ultraviolet laser light", Arch. ophthalmol. 103: 1741-1742, 1985). Excimer far UV radiation can be controlled to produce minimal adjacent tissue damage providing the angle and depth can be precisely controlled. The remaining problem of effects on healing could then be addressed.
Wound healing was assessed in rabbits following excimer laser surface ablation (Hanna, K. D., et al., "Corneal stromal wound healing in rabbits after 193 nm excimer laser surface ablation", Arch. Ophthalmol. 107: 895-901, 1989). Healing appeared to be excellent except when over 85% to 90% of the corneal thickness had been cut. Endothelial cell disruption, junction separation and individual cell dropout occurred with corneal haze development with the deeper cuts. A delivery system designed to deliver predictable depths of cut is, therefore, essential. Similar findings were reported in studies on human blind eyes (Taylor, D. M., et al., "Human excimer laser lamellar Keratectomy", Ophthalmology 96: 654-664, 1989). Attention was directed to the challenges of improved procedures and equipment, the problems of individual variation, and the control of biologic responses to trauma before excimer laser lamellar keratectomy could become a clinically useful means of correcting refractive errors. In living monkey eyes, it was concluded that mild, typical wound healing occurred after excimer laser keratomileusis (Fantes, F. E., et al., "Wound healing after excimer laser keratomileusis [photorefractive keratectomy] in monkeys", Arch. Ophthalmol. 108: 665-675, 1990). All corneas were epithelialized by 7 days. By 6 weeks, mild to moderate haze was apparent with clearing by 6 to 9 months. The epithelium was thickened at 21 days after ablation, but returned to normal by 3 months. Subepithelial fibroblasts were three times the density of normal keratocytes, but returned to nearly normal numbers by 9 months. One conclusion reached was that control of the contour and uniformity of the ablated surface is important for structural and biological responses of the cornea.
Review of the literature clearly reveals that far UV vaporization (ablation with an excimer laser at 193 nm, for example) is a feasible means to sculpture or reprofile the cornea to correct nearsightedness, farsightedness, astigmatism, corneal scars, corneal densities, etc. The healing appears to parallel or to be equal to healing after scalpel intervention, providing the proper guidelines for pulsing and duration are followed. There remains a need to control the contour and uniformity of the ablated surface. Such control will reduce the adverse structural and biological response of the cornea and insure that a desired corrective change results.
The use of a mask, of nearly identical optical density to the cornea, which can be preformed on the corneal surface so as to provide a smooth surface of exact contour and accurate dimensions would correct many of the problems that have prevented the precise control of the laser been during keratotomy. This mask would be required to withstand exposure to moist gases direct tangentally to the corneal surface throughout the duration of exposure to the laser to remove ablated debris. The modulation of the beam energy distribution of the laser in a controlled fashion should also be provided by such a corneal mask. The use of a smooth ablatable mask having a known contour and having the density of the cornea would aid in insuring accurate direction and depth of a tangental cut utilizing a laser beam. The ablatable mask of the invention provides such advantages.
Ionic polysaccharides have been used in the application of drugs by controlled release. Such ionic polysaccharides as chitosan or sodium alginate are disclosed as useful in providing spherical agglomerates of water-insoluble drugs in the Journal of Pharmaceutical Sciences, volume 78, number 11, November 1989, Bodmeier et al. Alginates have also been used as a depot substance in active immunization, as disclosed in the Journal of Pathology and Bacteriology, volume 77, (1959), C. R. Amies. Calcium alginate gel formulations have also found use as a matrix material for the controlled release of herbicides, as disclosed in the Journal of Controlled Release, 3 (1986) pages 229-233, Pfister et al. Alginates have also been used to form hydrogel foam wound dressings, as disclosed in U.S. Pat. No. 4,948,575.